A conventional laundry treatment apparatus includes a cabinet defining the external appearance thereof, a tub provided in the cabinet, a drum rotatably provided in the tub for washing laundry, and a motor having a shaft fixed to the drum via the tub for rotating the drum.
The drum may be rotated without dynamic equilibrium or dynamic balance depending on the position of the laundry in the drum.
Dynamic balance means the state in which the centrifugal force generated when a body of rotation is rotated or a moment generated by the centrifugal force becomes zero (0) with respect to the axis of rotation thereof. For a rigid body, dynamic balance is maintained if mass distribution is uniform about the axis of rotation thereof.
In the laundry treatment apparatus, therefore, dynamic balance may be understood as the case in which, when the drum is rotated in the state in which laundry is placed in the drum, the mass distribution of the laundry is within an allowable range about the axis of rotation of the drum (the case in which the drum is rotated while vibrating within an allowable range).
In the laundry treatment apparatus, on the other hand, dynamic unbalance is the state in which mass distribution is not uniform about the axis of rotation of the drum during the rotation of the drum, which occurs when laundry is not uniformly distributed in the drum.
When the drum is rotated in the unbalanced state thereof, the drum vibrates, and the vibration of the drum is transmitted to the tub or to the cabinet, whereby noise is generated.
There is a conventional laundry treatment apparatus including a balancing unit for solving the problem of unbalance of a drum. The balancing unit included in the conventional laundry treatment apparatus is a ball balancer or a fluid balancer having a ball or a fluid provided in a housing fixed to the drum.
The ball balancer or the fluid balancer included in the conventional laundry treatment apparatus is configured such that, when the rotational track of the drum is irregular due to laundry that causes unbalance, the ball or the fluid moves in the direction opposite the position at which the laundry that causes unbalance is placed to control the balance.
The above-described unbalance control is effective in a steady state, in which the vibration of the drum is within a predetermined range. However, the unbalance control is not effective in a transient state, in which the vibration of the drum is transient (transient vibration). In addition, the conventional balancing unit has a structure in which, when unbalance occurs, it is difficult to immediately solve the unbalance (i.e. to actively solve the unbalance).